The Cardiac Cycle
The term “diastole” refers to the normal postsystolic dilation of the heart cavities during which they full with blood. The term “systole” refers to contraction of the heart, especially of the ventricles, by which the blood is driven through the aorta and pulmonary artery to traverse the systemic and pulmonary circulations, respectively.
The term “cardiac cycle” is used to refer to all or any of the mechanical events related to the coronary blood flow or blood pressure that occurs from the beginning of one heartbeat to the beginning of the next. Blood pressure increases and decreases throughout the cardiac cycle. The frequency of the cardiac cycle is the heart rate. Every single ‘beat’ of the heart involves five major stages: (1)“Late diastole,” which is when the semilunar valves close, the atrioventricular (Av) valves open and the whole heart is relaxed; (2) “Atrial systole,” which is when the myocardium of the left and right atria are contracting, AV valves open and blood flows from atrium to ventricle; (3) “Isovolumic ventricular contraction,” which is when the ventricles begin to contract, AV and semilunar valves close, and there is no change in volume; (4) “ventricular ejection,” which is when the ventricles are empty but still contracting and the semilunar valves are open; and (5) “Isovolumic ventricular relaxation,” when pressure decreases, no blood is entering the ventricles, the ventricles stop contracting and begin to relax, and the semilunar valves are shut because blood in the aorta is pushing them shut. The cardiac cycle is coordinated by a series of electrical impulses that are produced by specialized heart cells found within the sino-atrial node and the atrioventricular node.
Coronary Blood Flow
The flow of blood through the coronary arteries is pulsatile, with characteristic phasic systolic and diastolic flow components. Systolic flow, which relates to the contraction or pumping phase of the heart cycle, has rapid, brief, retrograde responses. Diastolic flow, which relates to the relaxation or filling phase of the heart cycle, occurs during the relaxation phase after myocardial contraction, with an abrupt increase above systolic levels and a gradual decline parallel with that of aortic diastolic pressures. Intramural coronary blood volume changes during each heartbeat, with the myocardium accommodating the volume change brought about by muscular contraction. Coronary venous flow is out of phase with coronary arterial flow, occurring predominantly in systole and nearly absent during diastole.
For each heartbeat, blood pressure varies between systolic and diastolic pressures. The term “systolic pressure” refers to the peak pressure in the arteries, which occurs near the end of the cardiac cycle when the ventricles are contracting. The term “diastolic pressure” refers to the minimum pressure in the arteries, which occurs near the beginning of the cardiac cycle when the ventricles are filled with blood.
Coronary blood flow not only is phasic but also varies with the type of vessel and location in the myocardium. Coronary arterioles appear to have specialized regulatory elements along their length that operate “in series” in an integrated manner. A system of multiple functional “valves” permits fine control of the coronary circulation. The smallest arterioles dilate during metabolic stress, resulting in reduced microvascular resistance and increased myocardial perfusion. Stenosis or narrowing of a blood vessel produces resistance to blood flow related directly to the morphologic features of the stenosis. As the upstream ateriolar pressure decreases due to a fall in distending pressure across the stenosis, myogenic dilation of slightly larger arterioles upstream occurs and causes an additional decrease in resistance. Increased flow in the largest arterioles augments shear stress and triggers flow-mediated dilation, further reducing the resistance of this network.
The arterial and venous pulsatile flow characteristics of the heart are dependent on intramyocardial compliance. The term “compliance” refers to a measure of the tendency of a hollow organ to resist recoil toward its original dimensions upon removal of a distending or compressing force. The higher the compliance the more elastic the material. Compliance is calculated using the following equation, where ΔV is the change in volume, and ΔP is the change in pressure:
  C  =            Δ      ⁢                          ⁢      V              Δ      ⁢                          ⁢      P      
The capacity of the heart as a reservoir is controlled by resistance arterioles to coronary vascular inflow. Outlet resistance is related to intramural cardiac veins. The intramyocardial capillary resistance influences both arterial and venous responses but predominantly acts in concert with outlet resistance.
Approximately 75% of total coronary resistance occurs in the arterial system, which comprises conductance (R1), prearteriolar (R2) and arteriolar and intramyocardial capillary vessels (R3). Normal epicardial coronary arteries in humans typically are 0.3 to 5 mm in diameter, and do not offer appreciable resistance to blood flow. Normally, large epicardial vessel resistance (R1) is trivial until atherosclerotic obstructions compromise the lumen. Precapillary arterioles (R2), 100 to 500 μm in size) are resistive vessels connecting epicardial to myocardial capillaries and are the principal controllers of coronary blood flow. They contribute approximately 25% to 35% of total coronary resistance. Distal precapillary arteriolar vessels (<100 μm in diameter), the main site of metabolic regulation of coronary blood flow, are responsible for 40-50% of coronary flow resistance. The dense network of about 4000 capillaries per square millimeter ensures that each myocyte is adjacent to a capillary. Capillaries are not uniformly patent (meaning open; affording free passage), because precapillary sphincters regulate flow according to the needs of the myocardium.
Several conditions, such as left ventricular hypertrophy, myocardial ischemia, or diabetes, can impair the microcirculatory resistance (R3), blunting the maximal absolute increase in coronary flow in times of increased oxygen demand.
Ischemia
The myocardium depends almost entirely on aerobic metabolism, since oxygen stores in the heart are meager. Myocardial oxygen supply rises and falls in response to the oxygen (energy) demands of the myocardium. The term “autoregulation” refers to the ability to maintain myocardial perfusion at constant levels in the face of changing driving forces. Autoregulation maintains coronary perfusion at relatively constant levels over a wide range of mean aortic pressure. When aortic pressure exceeds its upper or lower limits, coronary blood flow precipitously declines or increases proportionately.
The heart needs to be supplied with a sufficient quantity of oxygen to prevent underperfusion. When reduced perfusion pressure distal to stenoses is not compensated by autoregulatory dilation of the resistance vessels, ischemia, meaning a lack of blood supply and oxygen, occurs. Because the zone least supplied generally is the farthest out, ischemia generally appears in areas farthest away from the blood supply.
After total or near-total occlusion of a coronary artery, myocardial perfusion occurs by way of collaterals, meaning vascular channels that interconnect epicardial arteries. Collateral channels may form acutely or may preexist in an under-developed state before the appearance of coronary artery disease. Preexisting collaterals are thin-walled structures ranging in diameter from 20 μm to 200 μm, with a variable density amoung different species. Preexisting collaterals normally are closed and nonfunctional, because no pressure gradient exists to drive flow between the arteries they connect. After coronary occlusion, the distal pressure drops precipitously and prexisting collaterals open virtually instantly.
The term “myocardial ischemia” refers to a decrease in blood supply and oxygen to the cells of the myocardium. The development of myocardial ischemia has been attributed to two mechanisms: (1) increased myocardial oxygen demand, and (2) decreased myocardial perfusion and oxygen delivery. (Willerson, J. T. et al, JACC 8(1): 245-50 (1986)). Myocardial ischemia generally appears first and is more extensive in the subendocardial region, since these deeper myocardial layers are farthest from the blood supply, with greater need for oxygen.
Transient ischemia, hibernating myocardium, and myocardial infarction are clinically different conditions.
Transient Ischemia. The term “transient ischemia” as used herein refers to a reversible (meaning that the myocytes survive the insult) narrowing of a coronary artery at rest or with exercise where there is no thrombus or plaque rupture but where blood supply cannot be met. Every time the heart's oxygen demand increases, an imbalance between oxygen demand and supply is created. Transient ischemia produces a cascade of events beginning with metabolic and biochemical alterations leading to impaired ventricular relaxation and diastrolic dysfunction, impaired systolic function, and electrocardiographic abnormalities with ST segment alterations, followed by increased end-diastolic pressure with left ventricular dyssynchrony, hypokinesis, akinesis, and dyskinesis, and lastly painful symptoms of angina. Even though ischemic myocytes experience physiological and metabolic changes within seconds of the cessation of coronary flow, resulting in T wave and sometimes ST segment abnormalities (but without serum enzyme elevation), no cell death results from the ischemia. Kloner, R. A. and Jennings, R B, Circulation 104: 2981-89 (2001). Once blood flow is re-established, a complete recovery of myocyte contractile function takes place.
Although angina pectoris (chest pain) may be a symptom of transient ischemia, by and large transient ischemia is silent (meaning ST-segment depression of at least 1 mm is present without associated symptoms, e.g., chest pain) in 79% of subjects. In most patients with stable angina, for example, physical effort or emotion, with a resultant increase in heart rate, blood pressure, or contractile state, or any combination thereof, increases myocardial oxygen demand without an adequate delivery in oxygen delivery through tightly narrowed (stenosed) coronary arteries. More than 40% of patients with stable angina treated with one or more antianginal drugs have frequent episodes of silent ischemia, which has been shown to predict a higher risk of coronary events and cardiac death. Deedwania, P C, Carbajal, E V, Arch. Intern. Med. 150: 2373-2382 (1991).
Chronic Myocardial Ischemia. The term “chronic myocardial ischemia (CMI)” as used herein refers to a prolonged subacute or chronic state of myocardial ischemia due to narrowing of a coronary blood vessel in which the myocardium “hibernates”, meaning that the myocardium downregulates or reduces its contractility, and hence its myocardial oxygen demand, to match reduced perfusion, thereby preserving cellular viability and evading apoptosis. The underlying mechanism by which the myocardium does so is poorly understood. This hibernating myocardium is capable of returning to normal or near-normal function on restoration of an adequate blood supply. Once coronary blood flow has been restored to normal or near normal and ischemia is resolved, however, the hibernating myocardium still does not contract. This flow-function mismatch resulting in a slow return of cardiac function after resolution of ischemia has been called stunning. The length of time for function to return is quite variable, ranging from days to months, and is dependent on a number of parameters, including the duration of the original ischemic insult, the severity of ischemia during the original insult, and the adequacy of the return of the arterial flowA number of studies have provided evidence for inflammation in hibernating myocardium. Heusch, G. et al., Am. J. Physiol. Heart Circ. Physiol. 288: 984-99 (2005). A study conducted in a porcine model of myocardial hibernation in which the mean rest LAD coronary blood flow was reduced to about 60% of baseline for a period of 24 hours to four weeks, detected apoptotic myocytes in all experimental pigs in the hibernating regions supplied by the stenotic LAD, suggesting that functional downregulation may not be adequate to prevent gradual, ongoing myocyte death through apoptosis in hibernating myocardium. Chen, C, et al., J. Am. Coll. Cardiol. 30: 1407-12 (1997). A biopsy study of human patients undergoing coronary artery bypass surgery, likewise recognized myocyte apoptosis, which negatively influences left ventricle functional recovery, as an important phenomenon in hibernating myocardium. Angelini, A., et al, Eur. J. Heart Failure 9(4): 377-83 (2006).
Acute Myocardial Infarction (AMI). Another type of insult occurs during AMI. AMI is an abrupt change in the lumen of a coronary blood vessel which results in ischemic infarction, meaning that it continues until heart muscle dies. On gross inspection, myocardial infarction can be divided into two major types: transmural infacts, in which the myocardial necrosis involves the full or nearly full thickness of the ventricular wall, and subendocardial (nontransmural) infarcts, in which the myocardial necrosis involves the subendocardium, the intramural myocardium, or both, without extending all the way through the ventricular wall to the epicardium. There often is total occlusion of the vessel with ST segment elevation because of thrombus formation within the lumen as a result of plaque rupture. The prolonged ischemic insult results in apoptotic and necrotic cardiomyocyte cell death. See Kajstura, J., et al., Lab Invest. 74: 86-107 (1996). Necrosis compromises the integrity of the sarcolemmal membrane and intracellular macromolecules such that serum cardiac markers, such as cardiac-specific troponins and enzymes, such as serum creatine kinase (CK), are released. In addition, the patient may have electrocardiogram (ECG) changes because of full thickness damage to the muscle. An ST-Elevation Myocardial Infarction (STEMI) is a larger injury than a non-ST-elevation myocardial infarction. ST-segment elevation and Q waves on the ECG, two features highly indicative of myocardial infarction, are seen in only about half of myocardial infarction cases on presentation.
Acute myocardial infarction remains common with a reported annual incidence of 1.1 million cases in the United States alone (Antman, E. M., Braunwald, E., Acute Myocardial Infarction, in Principles of Internal Medicine, 15th Ed., Braunwald, E. et al., Eds., New York: McGraw-Hill (2001)). Preclinical and clinical data demonstrate that following a myocardial infarction, the acute loss of myocardial muscle cells and the accompanying peri-infarct zone hypo-perfusion result in a cascade of events causing an immediate diminution of cardiac function, with the potential for long term persistence. The extent of myocardial cell loss is dependent on the duration of coronary artery occlusion, existing collateral coronary circulation and the condition of the cardiac microvasculature. Paul et al., Am. Heart J. 131: 710-15 (1996); Pfeffer, M. A., Braunwald, E., Circulation 81: 1161-72 (1990); Sheilban, I. et. al., J. Am. Coll. Cardiol. 38: 464-71 (2001); Braunwald E., Bristow, M. R., Circulation 102: IV-14-23 (2000); Rich et al., Am. J. Med. 92:7-13 (1992); Ren et al., J. Histochem. Cytochem. 49: 71-79 (2002); Hirai, T. et al., Circulation 79: 791-96 (1989); Ejiri, M. et al., J. Cardiology 20: 31-37 (1990). Because myocardial cells have virtually no ability to regenerate, myocardial infarction leads to permanent cardiac dysfunction due to contractile-muscle cell loss and replacement with nonfunctioning fibrotic scarring. Frangogiannis, N. G., et al., Cardiovascular Res. 53(1): 31-47 (2002). Moreover, compensatory hypertrophy of viable cardiac muscle leads to microvascular insufficiency that results in further demise in cardiac function by causing myocardial muscle hibernation and apoptosis of hypertrophied myocytes in the peri-infarct zone.
Among survivors of myocardial infarction, residual cardiac function is influenced by the extent of ventricular remodeling (meaning changes in size, shape, and function, typically a progressive decline in function, of the heart after injury). Alterations in ventricular topography (meaning the shape, configuration, or morphology of a ventricle) occur in both infarcted and healthy cardiac tissue after myocardial infarction. Pfeffer, M. A., Braunwald, E., Circulation 81: 1161-72 (1990). Ventricular dilatation (meaning a stretching, enlarging or spreading out of the ventricle) causes a decrease in global cardiac function and is affected by the infarct size, infarct healing and ventricular wall stresses. Recent efforts to minimize remodeling have been successful by limiting infarct size through rapid reperfusion (meaning restoration of blood flow) using thromobolytic agents and mechanical interventions, including, but not limited to, placement of a stent, along with reducing ventricular wall stresses by judicious use of pre-load therapies and proper after-load management. Id. Regardless of these interventions, a substantial percentage of patients experience clinically relevant and long-term cardiac dysfunction after myocardial infarction. Sheiban, I. et al., J. Am. Coll. Cardiol. 38: 464-71 (2001). Despite revascularization of the infarct related artery circulation and appropriate medical management to minimize ventricular wall stresses, a significant percentage of these patients experience ventricular remodeling, permanent cardiac dysfunction, and progressive deterioration of cardiac function, and consequently remain at an increased lifetime risk of experiencing adverse cardiac events, including death. Paul et al., Am. Heart J. 131: 710-15 (1996); Pfeffer, M. A., Braunwald, E., Circulation 81: 1161-72 (1990).
At the cellular level, immediately following a myocardial infarction, transient generalized cardiac dysfunction uniformly occurs. In the setting of a brief (i.e., lasting three minutes to five minutes) coronary artery occlusion, energy metabolism is impaired, leading to demonstrable cardiac muscle dysfunction that can persist for up to 48 hours despite immediate reperfusion. This so-called “stunned myocardium phenomenon” occurs subsequent to or after reperfusion and is thought to be a result of reactive oxygen species. The process is transient and is not associated with an inflammatory response. Frangogiannis, N. G., et al., Cardiovascular Res. 53(1): 31-47 (2002). After successful revascularization, significant recovery from stunning occurs within three to four days, although complete recovery may take much longer. Boli, R., Prog. Cardiovascular Disease 40(6): 477-515 (1998); Sakata, K. et al., Ann. Nucleic Med. 8: 153-57 (1994); Wollert, K. C. et al., Lancet 364: 141-48 (2004).
Coronary artery occlusion of more significant duration, i.e., lasting more than five minutes, leads to myocardial ischemia and is associated with a significant inflammatory response that begins immediately after reperfusion and can last for up to several weeks. Frangogiannis, N. G., et al., Cardiovascular Res. 53(1): 31-47 (2002); Frangogiannis, N. G. et al., Circulation 98: 687-798 (1998).
The inflammatory process following reperfusion is complex. Initially it contributes to myocardial damage but later leads to healing and scar formation. This complex process appears to occur in two phases. In the first so-called “hot” phase (within the first five days), reactive oxygen species (in the ischemic myocardial tissue) and complement activation generate a signal chemotactic for leukocytes (chemotaxis is the directed motion of a motile cell, organism or part towards environmental conditions it deems attractive and/or away from surroundings it finds repellent) and initiate a cytokine cascade. Lefer, D. J., Granger, D. N., Am. J. Med. 4:315-23 (2000); Frangogiannis, N. G., et al., Circulation 7:699-710 (1998). Mast cell degranulation, tumor necrosis factor alpha (TNFα) release, and increased interleukin-6 (IL-6), intercellular adhesion molecule 1 (“ICAM-1” or CD-54, a receptor typically expressed on endothelial cells and cells of the immune system), selectin (L, E and P) and integrin (CD11a, CD11b and CD18) expression all appear to contribute to neutrophil accumulation and degranulation in ischemic myocardium. Frangogiannis, N. G. et al., Circulation 7: 699-710 (1998), Kurrelmeyer, K. M, et al., Proc. Nat'l Acad. Sci. 10: 5456-61 (2000); Lasky, L. A., Science 258: 964-69 (1992); Ma, X. L., et al., Circulation 88(2): 649-58 (1993); Simpson, P. J. et al., J. Clin. Invest. 2: 624-29 (1998). Neutrophils contribute significantly to myocardial cell damage and death through microvascular obstruction and activation of neutrophil respiratory burst pathways after ligand-specific adhesion to cardiac myocytes. Entman, M. L., et al., J. Clin. Invest. 4: 1335-45 (1992). During the “hot” phase, angiogenesis is inhibited due to the release of angiostatic substances, including interferon gamma-inducible protein (IP 10). Frangogiannis, N. G., et al., FASEB J. 15: 1428-30 (2001).
In the second phase, the cardiac repair process begins (about day 6 to about day 14), which eventually leads to scar formation (about day 14 to about day 21) and subsequent ventricular remodeling (about day 21 to about day 90). Soon after reperfusion, monocytes infiltrate the infarcted myocardium. Attracted by complement (C5a), transforming growth factor B1 (“TGF-B1”) and monocyte chemotactic protein 1 (“MCP-1”), monocytes differentiate into macrophages that initiate the healing process by scavenging dead tissue, regulating extracellular matrix metabolism, and inducing fibroblast proliferation. Birdshall, H. H., et al., Circulation 3: 684-92 (1997). Secretion of interleukin 10 (IL-10) by infiltrating lymphocytes also promotes healing by down-regulating inflammatory cytokines and influencing tissue remodeling. Frangogiannis, N. G. et al., J. Immunol. 5:2798-2808 (2000). Mast cells also appear to be involved in the later stages of myocardial repair by participating in the formation of fibrotic scar tissue. Stem Cell Factor (SCF) is a potent attractor of mast cells. SCF mRNA has been shown to be up-regulated in ischemic myocardial segments in a canine model of myocardial infarction and thus may contribute to mast cell accumulation at ischemic myocardial sites. Franigogiannis, N. G. et al., Circulation 98: 687-798 (1998). Mast cell products (including TGF-B, basic fibroblast growth factor (bFGF), vascular endothelial growth factor (VEGF) and gelatinases A and B) induce fibroblast proliferation, influence extracellular matrix metabolism, and induce angiogenesis. Fang, K. C., et al., J. Immunol. 162: 5528-35 (1999); Takeshi, S., et al., Cardiology 93: 168-74 (2000).
Following a myocardial infarction, neoangiogenesis occurs after the “hot” phase of the inflammatory process subsides (about day 5) coincident with rising levels of VEGF (VEGF peaks at about day 7 and gradually subsides to baseline at about day 14 to about day 21). During this phase of the healing process, endothelial precursor cells (EPCs) are mobilized and recruited to the infarct site. Shinitani, S., et al., Circulation 103: 2776-79 (2001). Without being limited by theory, it has been suggested that the chemokine stromal cell derived factor-1 (SDF-1), which is the ligand for the CXCR-4 chemokine receptor expressed by CD34+ cells, also plays a role in homing of cells to areas of ischemic damage. Ceredini, D. J., et al., Nature Medicine 10: 858-63 (2004); Askari, A., et al., Lancet 362: 697-703 (2003); Yamaguchi, J. et al., Circulation 107: 1322-34 (2003). While it is known that SDF-1 plays a role in hematopoiesis and is involved in migration, homing and survival of hematopoietic progenitors, and while SDF-1 has been implicated in ischemic neovascularization in vivo by augmenting EPC recruitment to ischemic sites (Yamaguchi et al. Circulation 107:1322-1328 (2003), SDF-1's role in neoangiogenesis is not certain. There is suggestive evidence implicating SDF-1. For example, SDF-1 gene expression is upregulated during hypoxia, a deficiency of oxygen in the tissues, by hypoxia inducible factor-1 (HIF-1). Furthermore, CD34+ cells are capable of homing to areas of ischemia, rich in SDF-1, including infarcted myocardium. Askari et al., Lancet 362: 697-703 (2003). Moreover, virtually all cells that express VEGF-2 coexpress CD34 and CXCR-4, but only about 1% to about 2% of CD34+CXCR-4+ cells co-express VEGF-2.
The Peri-Infarct Border Zone
The zone of dysfunctional myocardium produced by coronary artery occlusion extends beyond the infarct region to include a variable boundary of adjacent normal appearing tissue. (Hu, Q., et al., Am. J. Physiol. Heart Circ. Physiol. 291: H648-657 (2006)). This ischemic, but viable, pen-infarct zone of tissue separates the central zone of progressive necrosis from surrounding normal myocardium. The peri-infarct zone does not correlate with enzymatic parameters of infarct size and is substantially larger in small infarcts. Stork, A., et al., European Radiol. 16(10): 2350-57 (2006).
Ischemia due to edema and compression of the blood vessels in the border zone may be very important to outcome. It is known, for example, that after an AMI, transient ischemia occurs in the border zones and that percutaneous coronary interventions, which open up the infarct-related artery, can adversely affect the health of the pen-infarct border zones. It has been suggested that intermediate levels of mean blood flow can exist as the result of admixture of peninsulas of ischemic tissue intermingled with regions of normally perfused myocardium at the border of an infarct. (Hu, Q., et al., Am. J. Physiol. Heart Circ Physiol. 291: H648-657 (2006)). However, the boundary of the intermingled coronary microvessels, which in dogs is no more than 3 mm in width, cannot explain the relatively broad region of dysfunctional myocardium surrounding an infarct. Murdock, R H, Jr., et al., Cir. Res. 52: 451-59 (1983); Buda, A J, et al., J. Am. Coll. Cariol. 8: 150-58 (1986). Progressive dysfunction of this pen-infarct myocardium over time may contribute to the transition from compensated remodeling to progressive heart failure after an AMI.
To date, no ideal therapy exists for preventing the long term adverse consequences of vascular insufficiency, particularly the significant vascular insufficiency after a myocardial infarction. While large vessel revascularization (meaning the successful placement of a stent) seems promising, studies to date have shown such applications to be insufficient in addressing increased demands posed by compensatory myocardial hypertrophy. As a result, infarct extension and fibrous replacement commonly occur, regardless of large vessel revascularization, appropriate medical management of ventricular wall stresses, and potential natural, albeit suboptimal, CD34+ cell-mediated neoangiogenesis (one of the theories relating to the underlying cause of myocardial infarction is that the ability to mobilize these cells may be biologically limited).
Intense interest has developed in evaluating the ability of endothelial and myocardial precursor cells to limit damage to the myocardium after infarction and to limit or prevent ventricular remodeling. Significant preclinical data and some clinical data demonstrate the safety and potential of cell therapy using a variety of cell precursors (particularly hematopoietic cells) to contribute to neoangiogenesis, limited cardiac myogenesis (principally by fusion), and muscle preservation in the myocardial infarct zone. See, e.g., Jackson, et al., J. Clin. Invest. 107: 1395-1402 (2001); Edelberg, J. M., et al., Cir. Res. 90: e89-e93 (2002); Schichinger, V. et al., New Engl. J. Med. 355 (12): 1210-21 (2006) (using bone marrow-derived progenitor cells); Assmus, B. et al., New Engl. J. Med. 355 (12) 1222-32 (2006) (using bone marrow-derived progenitor cells), but see Lunde, K. et al., New Eng. J. Med. 355 (12): 1199-209 (2006) (using bone marrow-derived progenitor cells). It is not known under what circumstances or the extent to which left ventriclar remodeling is reversible.
Bone marrow consists of a variety of precursor and mature cell types, including hematopoietic cells (the precursors of mature blood cells) and stromal cells (the precursors of a broad spectrum of connective tissue cells), both of which appear to be capable of differentiating into other cell types. Wang, J. S. et al., J. Thorac. Cardiovasc. Surg. 122; 699-705 (2001); Tomita, S. et al., Circulation 100 (Suppl. II): 247-256 (1999); Saito, T. et al., Tissue Eng. 1: 327-43 (1995). Unmodified (i.e., not fractionated) marrow or blood-derived cells have been used in several clinical studies, for example, Hamano, K. et al., Japan Cir. J. 65: 845-47 (2001); Strauer, B. E., et al., Circulation 106: 1913-18 (2002); Assmus, et al., Circulation 106: 3009-3017 (2002); Dobert, N. et al., Eur. J. Nuel. Med. Mol. Imaging, 8: 1146-51 (2004); Wollert, K. C. et al., Lancet 364: 141-48 (2004). Since the mononuclear fraction of bone marrow contains stromal cells, hematopoietic precursors, and endothelial precursors, the relative contribution of each of these populations to the observed effects, if any, remains unknown.
CD34 is a hematopoietic stem cell antigen selectively expressed on hematopoietic stem and progenitor cells derived from human bone marrow, blood and fetal liver. Yin et al., Blood 90: 5002-5012 (1997); Miaglia, S. et al., Blood 90: 5013-21 (1997). Cells that express CD34 are termed CD34+. Stromal cells do not express CD34 and are therefore termed CD34−. CD34+ cells isolated from human blood may be capable of differentiating into cardiomyocytes, endothelial cells, and smooth muscle cells in vivo. See Yeh, et al., Circulation 108: 2070-73 (2003). CD34+ cells represent approximately 1% of bone marrow derived nucleated cells; CD34 antigen also is expressed by immature endothelial cell precursors, while mature endothelial cells do not express CD34. Peichev, M. et al., Blood 95: 952-58 (2000). In vitro, CD34+ cells derived from adult bone marrow give rise to a majority of the granulocyte/macrophage progenitor cells (CFU-GM), some colony-forming units-mixed (CFU-Mix) and a minor population of primitive erythroid progenitor cells (burst forming units, erythrocytes or BFU-E). Yeh, et al., Circulation 108: 2070-73 (2003). CD34+ cells also may have the potential to differentiate into, or to contribute to, the development of new myocardial muscle, albeit at low frequency.
Techniques have been developed using immunomagnetic bead separation to isolate a highly purified and viable population of CD34+ cells from bone narrow mononuclear cells. See U.S. Pat. Nos. 5,536,475, 5,035,994, 5,130,144, 4,965,205, the contents of each of which is incorporated herein by reference. Two clinical studies support the clinical application of bone marrow derived CD34+ cells after myocardial infarction. See C. Stamm, et al., Lancet 361: 45-46 (2003); Herenstein, B. et al., Blood Supplement, Abs. 2696 (2004).
Animal Models
A preferred therapy after AMI would stop cells from dying during recovery that leads to reverse remodeling and failure, or replace the dying cells with cardiomyocytes.
A conditional transgenic system that allows control of the timing of VEG-F expression has been described in mice. May, D et al, Proc. Nat'l Acad. Sci. 105(1): 282-87 (2008). The system was used to create a tunable state of ventricular hypoperfusion and myocardial ischemia. Under conditions where a large fraction of cardiomyocytes are driven to enter the hibernation mode yet without a detectable cell death, cardiomyocyte dysfunction (hibernation) was found to be linearly related to the extent of reduction in microvascular density. This does not model AMI-induced ventricular remodeling, which is complicated by processes of infarct expansion, inflammation, scar formation and myocyte hypertrophy.
Peripheral artery disease (PAD), also called peripheral vascular disease (PVD), is modeled by the hind limb model of ischemia in which the femoral artery of the mouse is tied off to simulate peripheral artery disease. PAD, which commonly affects the arteries supplying the leg and includes all diseases caused by the obstruction of large arteries in the arms and legs, can result from atherosclerosis, inflammatory processes leading to stenosis, an embolism or thrombus formation. Restriction of blood flow due to arterial stenosis or occlusion often leads patients to complain of muscle pain on walking (intermittent claudication). Any further reduction in blood flow causes ischemic pain at rest. This condition is called chronic limb ischemia, meaning the demand for oxygen cannot be sustained when resting. Ulceration and gangrene may then supervene in the toes, which are the furthest away from the blood supply, and can result in loss of the involved limb if not treated.
Therapies for limb ischemia have the goals of collateral development and blood supply replenishment. Bone marrow derived CD34+ mononuclear cells have been tested in such hindlimb ischemia models, but the hindlimb ischemia model does not model what takes place in the heart.
The closest animal model, the pig model, is not a good model of human disease because (i) all experiments generally are done in nonatherosclerotic animals, (ii) the animals are not treated with angioplasty, (iii) normal pigs do not embolize blood vessels; (iv) circulation of the pig is not exactly the same as human; and (iv) the peri-infarct border zone may not be the same.
A marginal improvement in angina symptoms recently was reported when CD34+ cells were mobilized with GCSF, apheresed after 5 days, and then injected into an ischemic area of the heart based on Noga mapping.
The described invention is a therapy for improving infarct-area perfusion after myocardial infarction. Data from a phase I trial has provided evidence that subjects treated with at least 10×106 isolated autologous CD34+ hematopoietic stem cells containing a subpopulation of at least 0.5×106 potent CD34+ cells expressing CXCR-4 and having CXCR-4 mediated chemotactic activity experienced significant improvement in resting perfusion rates at 6 months compared to subjects receiving 5 million cells and control, as measured by the SPECT Total Severity Score (−256 versus +13, p=0.01).